E. coli bacteria grew in 15NH4Cl medium for several generations are allowed to grow in 14NH4Cl medium. After 2 generations, the bacteria are isolated from the medium and DNA of bacteria centrifuged in CsCI. The result of the density gradient of DNA is
(1) both heavy and light DNA
(2) only hybrid DNA
(3) both hybrid and light DNA
(4) both hybrid and heavy DNA
